The invention relates to a method for sintering aluminium-based sintered parts whereby the following steps are carried out in separate atmospheres in spatially separate areas in each case:
a) the parts to be sintered are de-bindered;
b) the parts to be sintered are heated to sintering temperature and maintained at this temperature for a certain time;
c) the sintered parts are cooled in a controlled manner,
and to a device for sintering aluminium-based sintered parts comprising
a) a de-bindering area in which the sintered parts are stripped of binding agents by heating;
b) a sintering area in which the parts to be sintered are subjected to a sintering process by heating to sintering temperature, which area has suitable heating arrangements for this purpose;
c) a cooling area in which the sintered parts can be cooled in a controlled manner after the sintering process;
d) a transport system which continuously conveys the parts for sintering through the different areas;
e) airlocks which keep the atmospheres of the different areas separate and through which the parts for sintering must pass on leaving a particular area.
In view of the positive properties inherent in aluminium the sintering of this metal is gaining increasing importance in very diverse technical fields, but in particular in motor vehicle construction. In the last-mentioned field weight-saving, which is associated with the use of aluminium, plays an especially important part.
In general, pure aluminium powder is not processed; rather, powder mixtures or alloyed powders containing in particular silicon as an additive are preferably used. All powders containing aluminium as an important constituent are here collectively called xe2x80x9caluminium-basedxe2x80x9d; such powders are in danger of forming oxides during sintering. Sintered aluminium parts with a relatively high silicon content are especially desired. However, with increasing silicon content the sintering process becomes more difficult. A further difficulty in sintering aluminium-based powders is that they require a higher content of binding agents during the pressing process. Whereas such binding agents, which are used at the same time as lubricants for the pressing tool, represent a content of approx. 0.7 to 1.0 weight percent in the sintering of iron, for example, binding agents representing a weight percentage of approx. 1.0 to 1.5 must be added when sintering aluminium. These binding agents must be completely removed before the sintering process. Altogether, the requirements for accuracy, reproducibility and homogeneity of temperature distribution are far more critical when sintering aluminium-based powder than when sintering other powders, in particular iron. For this reason sintered aluminium parts have not yet come into use in all cases where this would in itself be desirable.
A method and a device of the above-mentioned type are described in DE-PS 197 19 203. Although the title of this document refers to a sintering process for pressed metal powder parts, which linguistically would also include aluminium powder, this method and device are in fact intended only for sintering iron-based powders since the fast cooling of the sintered parts below the xe2x80x9cmartensite start linexe2x80x9d claimed in that document is only conceivable for such powders.
It is the object of the present invention to specify a method of the above-mentioned type whereby high-grade aluminium-based sintered parts can be manufactured.
This object is achieved according to the invention in that in process step b) an inert gas the oxygen content of which corresponds to a dew point not higher than xe2x88x9240xc2x0 C. is used as the atmosphere, and in that the parts for sintering are heated to a sintering temperature of 560-620xc2x0 C. through circulation of the correspondingly heated inert gas.
The invention is therefore based on a recognition of two factors: because an upper limit is placed on the oxygen content of the inert atmosphere it is ensured that no undesired oxides which would detrimentally influence the product of sintering can form in the sintering process; and because, unlike the subject of the above-mentioned DE-PS 197 19 203, the parts for sintering are heated not by radiant heat but by convection heat, for which purpose the high-purity inert gas is impelled in a circulating current, the heating of the parts for sintering has a homogeneity which could not otherwise be achieved. The desired high quality of the sintered products results only from the combination of these features.
Nitrogen is preferably used as the inert gas. This gas is commercially obtainable at the required purity and is very much cheaper than noble gases which in principle could also be used.
A further object of the present invention is so to configure a device of the above-mentioned type that it is suitable for the manufacture of high-grade aluminium-based sintered parts.
This object is achieved according to the invention in that
f) the atmosphere in the sintering area is formed by an inert gas the oxygen content of which corresponds to a dew point not higher than xe2x88x9240xc2x0 C.;
g) the sintering area comprises at least one heating arrangement for the parts for sintering which includes indirectly heated heat exchanging surfaces, a fan and an air guidance arrangement such that a circulating flow of the inert gas around the parts for sintering can be induced.
The purpose of these features and the advantages attainable thereby coincide with what was said above regarding the method according to the invention.
The advantages of the embodiment specified in claims 4 and 5 have also already been indicated above with reference to the method according to the invention.
The sintering area of a sintering device must be of a length which corresponds to the time needed for sintering at the selected transport speed. In general, it is recommended that a relatively long sintering area comprises a plurality of zones separated by dividing walls, each of which zones has a heating arrangement with heat exchanging surfaces, a fan and an air guidance arrangement. In this way uniformly defined gas flow characteristics can be established at all points even in the case of relatively long sintering areas.
In particular in the heating zone of the sintering area the temperature of the inert gas differs between zones of the sintering area located successively in the direction of movement.
The embodiment of the invention in which the gas circulation around the parts for sintering differs in successive zones of the sintering area in the direction of movement yields especially good sintering results because of the highly homogenous temperature profile. For example, the parts for sintering can be exposed to a gas flow in one case from below to above, in another case from above the below, in another case to a flow rotating clockwise in the direction of movement and in another case to a gas flow rotating anticlockwise in the direction of movement.
It is also advantageous if a nozzle plate is provided, by means of which the circulating inert gas is directed against the parts for sintering. The gas flow in the area of the parts for sintering and therefore the heating undergone by said parts can thereby be further homogenised.
As mentioned above, careful maintenance of the purity of the atmosphere in the sintering area plays an especially important part. Accordingly, special attention must be paid to gas sealing during the transfer of the parts for sintering to and from the sintering area. It is especially preferred if the doors of the airlocks adjacent the inlet and/or the outlet of the sintering area are closable in a not completely sealed manner and if the inert gas in the sintering area is at a pressure above atmospheric. Through the intentional xe2x80x9cleakagexe2x80x9d in the door of the airlock adjacent the sintering area a flushing current of inert gas from the sintering area into the airlock concerned is constantly maintained, by which flow, firstly, the interior of the airlock is flushed and, secondly, penetration of external atmosphere from the airlock into the sintering area is prevented.
An embodiment of the invention is explained in more detail below with reference to the drawings, in which:
FIG. 1 represents schematically a sintering furnace for sintering aluminium-based sintered parts;
FIG. 2 shows a cross-section through the sintering furnace of FIG. 1 in the area of the sintering zone on an enlarged scale.